Radio communication system, radio base station apparatus, user terminal and radio communication method

ABSTRACT

To reduce overhead of CSI feedback without decreasing throughput in applying CoMP transmission, a radio communication method of the invention is characterized in that a radio base station apparatus determines so that content of CSI to transmit to a serving cell to perform CoMP transmission as feedback is different from content of CSI to transmit to a coordinated cell as feedback, and notifies a user terminal of the content of CSI, and that the user terminal generates feedback information based on the content of CSI notified from the radio base station apparatus, and transmits the feedback information to radio base station apparatuses of cells that perform Coordinated Multi-Point transmission.

TECHNICAL FIELD

The present invention relates to a radio communication system, radiobase station apparatus, user terminal and radio communication methodapplicable to a cellular system and the like.

BACKGROUND ART

In UMTS (Universal Mobile Telecommunications System) networks, for thepurpose of improving spectral efficiency and further improving datarates, by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA(High Speed Uplink Packet Access), it is performed exploiting maximumfeatures of the system based on W-CDMA (Wideband-Code Division MultipleAccess). For the UMTS network, for the purpose of further increasinghigh-speed data rates, providing low delay and the like, Long TermEvolution (LTE) has been studied (Non-patent Literature 1).

In the 3G system, a fixed band of 5 MHz is substantially used, and it ispossible to achieve transmission rates of approximately maximum 2 Mbpsin downlink. Meanwhile, in the LTE system, using variable bands rangingfrom 1.4 MHz to 20 MHz, it is possible to achieve transmission rates ofmaximum 300 Mbps in downlink and about 75 Mbps in uplink. Further, inthe UMTS network, for the purpose of further increasing the wide-bandand high speed, successor systems to LTE have been studied (for example,LTE Advanced (LTE-A)).

CITATION LIST

Non-Patent Literature

-   [Non-patent Literature 1] 3GPP, TR25.912 (V7.1.0), “Feasibility    study for Evolved UTRA and UTRAN”, September 2006

SUMMARY OF THE INVENTION Technical Problem

In addition, as one of promising techniques to further improve systemperformance of LTE system, there is inter-cell orthogonalization. Forexample, in the LTE-A system, intra-cell orthogonalization is achievedby orthogonal multiple access both in uplink and downlink. In otherwords, in downlink, user terminals UEs (User Equipments) areorthogonalized in the frequency domain. Meanwhile, for inter-cell,interference randomizing by 1-cell frequency reuse is a base as inW-CDMA.

Therefore, the 3GPP (3rd Generation Partnership Project) has studiedCoordinated Multi-Point transmission/reception (CoMP) as techniques foractualizing inter-cell orthogonalization. In CoMPtransmission/reception, a plurality of cells coordinates to performsignal processing of transmission and reception on a single or aplurality of user terminals UEs. For example, in downlink, studied areplurality-of-cell simultaneous transmission applying precoding,Coordinated Scheduling/Beamforming and the like. By applying these CoMPtransmission/reception techniques, it is expected to improve throughputcharacteristics of user terminals UEs particularly positioned at thecell edge.

To apply CoMP transmission/reception techniques, it is necessary that auser terminal transmits channel state information (CSI) on a pluralityof cells to a radio base station apparatus as feedback. Therefore,overhead of CSI feedback is large. When overhead of CSI feedback issimply decreased, it is not possible to effectively apply CoMPtransmission/reception techniques, and throughput is not improved.

The present invention was made in view of such respects, and it is anobject of the invention to provide a radio communication system, radiobase station apparatus, user terminal and radio communication methodthat enable overhead of CSI feedback to be reduced without decreasingthroughput in applying CoMP transmission.

Solution to Problem

A radio communication system of the present invention is a radiocommunication system provided with a plurality of radio base stationapparatuses, and a user terminal configured to be able to performCoordinated Multi-Point transmission/reception with the plurality ofradio base station apparatuses, and is characterized in that the radiobase station apparatus includes a determination section that determinesso that content of channel state information to transmit to a servingcell to perform Coordinated Multi-Point transmission as feedback isdifferent from content of channel state information to transmit to acoordinated cell as feedback, and a notification section that notifies auser terminal of the content of channel state information, and that theuser terminal includes a generation section that generates feedbackinformation based on content of channel state information notified fromthe radio base station apparatus, and a transmission section thattransmits the feedback information to radio base station apparatuses ofcells that perform Coordinated Multi-Point transmission.

A radio base station apparatus of the invention is a radio base stationapparatus in a radio communication system provided with a plurality ofradio base station apparatuses, and a user terminal configured to beable to perform Coordinated Multi-Point transmission/reception with theplurality of radio base station apparatuses, and is characterized byincluding a determination section that determines so that content ofchannel state information to transmit to a serving cell to performCoordinated Multi-Point transmission as feedback is different fromcontent of channel state information to transmit to a coordinated cellas feedback, and a notification section that notifies a user terminal ofthe content of channel state information.

A user terminal of the invention is a user terminal in a radiocommunication system provided with a plurality of radio base stationapparatuses, and a user terminal configured to be able to performCoordinated Multi-Point transmission/reception with the plurality ofradio base station apparatuses, and is characterized by including ageneration section that generates feedback information based on contentof channel state information notified from the radio base stationapparatus, and a transmission section that transmits the feedbackinformation to radio base station apparatuses of cells that performCoordinated Multi-Point transmission.

A radio communication method of the invention is a radio communicationmethod in a radio communication system provided with a plurality ofradio base station apparatuses, and a user terminal configured to beable to perform Coordinated Multi-Point transmission/reception with theplurality of radio base station apparatuses, and is characterized byincluding the steps of, in the radio base station apparatus, determiningso that content of channel state information to transmit to a servingcell to perform Coordinated Multi-Point transmission as feedback isdifferent from content of channel state information to transmit to acoordinated cell as feedback, and notifying a user terminal of thecontent of channel state information, and, in the user terminal,generating feedback information based on the content of channel stateinformation notified from the radio base station apparatus, andtransmitting the feedback information to radio base station apparatusesof cells that perform Coordinated Multi-Point transmission.

Technical Advantage of the Invention

According to the present invention, it is possible to reduce overhead ofCSI feedback without decreasing throughput in applying CoMPtransmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 contains diagrams to explain Coordinated Multi-Pointtransmission;

FIG. 2 contains schematic diagrams illustrating a configuration of aradio base station apparatus applied to Coordinated Multi-Pointtransmission;

FIG. 3 is a table showing combinations of CSI contents of a serving celland a coordinated cell;

FIG. 4 is a sequence diagram illustrating a procedure between a radiobase station apparatus and a user terminal;

FIG. 5 is a diagram to explain a system configuration of a radiocommunication system;

FIG. 6 is a diagram to explain an entire configuration of the radio basestation apparatus;

FIG. 7 is a functional block diagram corresponding to a basebandprocessing section of the radio base station apparatus;

FIG. 8 is a diagram to explain an entire configuration of a userterminal; and

FIG. 9 is a functional block diagram corresponding to a basebandprocessing section of the user terminal.

DESCRIPTION OF EMBODIMENTS

An Embodiment of the present invention will specifically be describedbelow with reference to accompanying drawings.

Downlink CoMP transmission will be described first with reference toFIG. 1. As downlink CoMP transmission, there are CoordinatedScheduling/Coordinated Beamforming and Joint processing. CoordinatedScheduling/Coordinated Beamforming is a method for transmitting a shareddata channel to one user terminal UE from only one cell, where radioresources in the frequency/spatial domain are allocated withconsideration given to interference from another cell and interferenceto another cell as shown in FIG. 1A. Meanwhile, Joint processing is amethod for transmitting shared data channels from a plurality of cellsat the same time by applying precoding, and has Joint transmission inwhich a plurality of cells transmits shared data channels to one userterminal UE as shown in FIG. 1B, and Dynamic Point Selection (DPS) inwhich one cell is instantaneously selected and transmits a shared datachannel as shown in FIG. 1C.

As a configuration to actualize CoMP transmission/reception, forexample, there are a configuration (centralized control based on the RREconfiguration) including a plurality of remote radio equipments (RREs)connected to a radio base station apparatus (radio base stationapparatus eNB) with optical fibers or the like as shown in FIG. 2A, andanother configuration (autonomous decentralized control based on theindependent base station configuration) of a radio base stationapparatus (radio base station apparatus eNB) as shown in FIG. 2B. Inaddition, FIG. 2A illustrates the configuration including a plurality ofremote radio equipments RREs, and may be of a configuration includingonly a single remote radio equipment RRE as shown in FIG. 1.

In the configuration (RRE configuration) as shown in FIG. 2A, the radiobase station apparatus eNB controls the remote radio equipments RRE1 andRRE2 in a centralized manner. In the RRE configuration, since the radiobase station apparatus eNB (centralized base station) that performsbaseband signal processing and control of a plurality of remote radioequipments RREs and each cell (i.e. each remote radio equipment RRE) areconnected with baseband signals using optical fibers, the centralizedbase station is capable of collectively performing radio resourcecontrol among cells. In other words, the problems of delay of signalingbetween radio base station apparatuses eNBs and overhead are small,which become problems in the independent base station configuration, andhigh-speed radio resource control among cells is relatively made ease.Accordingly, in the RRE configuration, in downlink, it is possible toapply the method using high-speed inter-cell signal processing such asplurality-of-cell simultaneous transmission.

Meanwhile, in the configuration (independent base station configuration)as shown in FIG. 2B, each of a plurality of radio base stationapparatuses eNBs (or RREs) performs radio resource allocation controlsuch as scheduling. In this case, by using an X2 interface between theradio base station apparatus eNB of a cell 1 and the radio base stationapparatus eNB of a cell 2, when necessary, timing information and radioresource allocation information such as scheduling is transmitted to anyone of the radio base station apparatuses eNBs to perform coordinationbetween cells.

CoMP transmission is applied to improve throughput of a user terminalexisting at the cell edge. Therefore, it is controlled to apply CoMPtransmission when a user terminal exists at the cell edge. In this case,a radio base station apparatus obtains a difference of qualityinformation (for example, RSRP (Reference Signal Received Power)) foreach cell from a user terminal, and in the case where the difference isa threshold or less i.e. in the case where the quality differencebetween cells is small, determines that the user terminal exists at thecell edge to apply CoMP transmission. Meanwhile, in the case where thedifference of quality information for each cell exceeds the thresholdi.e. in the case where the quality difference between cells is large,since the user terminal is close to a radio base station apparatus ofeither cell, the radio base station apparatus determines that the userterminal exists near the center of the cell, and does not apply CoMPtransmission.

In the case of applying CoMP transmission, the user terminal transmitschannel state information for each of a plurality of cells to the radiobase station apparatus (radio base station apparatus of the servingcell) as feedback. Meanwhile, in the case of not applying CoMPtransmission, the user terminal transmits the channel state informationof the serving cell to the radio base station apparatus as feedback.Thus, when CoMP transmission is applied, since the user terminaltransmits the channel state information for each of a plurality of cellsas feedback, overhead of feedback information increases.

Herein, as the channel state information, there is channel stateinformation for each cell, and inter-cell channel state information. InJoint transmission type CoMP, radio base station apparatuses of aplurality of cells transmit the same data with the same phase to a userterminal at the same timing. In this case, it is necessary to transmitinter-cell information as feedback so that the user terminal is capableof receiving the same data with the same phase at the same timing,together with the channel state information of each cell. As the channelstate information for each cell, there are PMI (Precoding MatrixIndicator), CDI (Channel Distribution Information), CQI (Channel QualityIndicator) and the like, and as the inter-cell channel stateinformation, there is phase difference information, amplitude differenceinformation and the like.

The inventors of the present invention noted content of channel stateinformation (CSI) in transmitting the above-mentioned channel stateinformation as feedback. In applying CoMP transmission, informationamounts notified from a user terminal are different between a servingcell (cell to which the user terminal is connected) and a coordinatedcell (cell which coordinates with the serving cell in CoMP). Since theserving cell is connected to the user terminal, the information amountnotified from the user terminal is large. Therefore, accuracy offeedback from the user terminal to the serving cell is required to behigh. Meanwhile, for the coordinated cell, since the information amountnotified from the user terminal is small, it is conceivable that it isnot necessary to make feedback accuracy equal to that of the servingcell. Therefore, the inventors of the invention found out that it ispossible to reduce overhead of feedback of the channel state informationby making content of channel state information to transmit to a servingcell as feedback different from content of channel state information totransmit to a coordinated cell as feedback.

Further, the inventors of the invention actually measured how much therelative capacity changes due to the difference (quality difference) inthe quality information for each cell, by changing content of inter-cellchannel state information. At this point, content of CSI of a servingcell was set so that feedback accuracy was high, and content of CSI of acoordinated cell was set so that feedback accuracy was low. As a result,it is understood that the relative capacity was kept high. Further, whenthe quality difference is relatively large, it is understood that it ispossible to obtain almost the same relative capacity as that in the caseof setting content of CSI of a serving cell and content of CSI of acoordinated cell both so that feedback accuracy is high. In other words,when the quality difference is relatively large, it is understood thatit is possible to maintain the relative capacity even by setting contentof CSI of a coordinated cell so that feedback accuracy is low.

Therefore, the inventors of the invention found out that it is possibleto reduce overhead of the channel state information to feed back bymaking content of CSI of a serving cell and content of CSI of acoordinated cell different from each other.

In other words, it is the gist of the invention to determine content ofCSI to transmit to a serving cell to perform CoMP transmission asfeedback and content of CSI to transmit to a coordinated cell asfeedback to be different from each other to perform feedback, and tothereby reduce overhead of CSI feedback without decreasing throughput inapplying CoMP transmission. Particularly, by determining content ofchannel state information so that accuracy of feedback to the servingcell is higher than accuracy of feedback to the coordinated cell, it ispossible to reduce overhead of CSI feedback without decreasingthroughput.

In the present invention, the content of channel state information isdetermined so that accuracy of feedback to a serving cell is higher thanaccuracy of feedback to a coordinated cell. By this means, it ispossible to reduce overhead of channel state information (CSI) feedback.Further, in the invention, the content of CSI is defined by (1)short-term covariance matrix, (2) fine quantization channel matrix, (3)long-term covariance matrix, (4) precoding matrix indicator (PMI) or (5)rough quantization channel matrix. Herein, (1) the short-term covariancematrix is a covariance matrix at a subframe level or sub-band level. (2)The fine quantization channel matrix is a channel matrix of quantization(for example, the number of bits higher than the existing PMI) finerthan granularity defined by the PMI. (3) The long-term covariance matrixis a covariance matrix of about several tens of milliseconds in theentire band. (5) The rough quantization channel matrix is a channelmatrix of quantization (for example, the number of bits lower than theexisting PMI) rougher than granularity defined by the PMI.

As described above, in the case of defining CSI with different feedbackaccuracy, feedback accuracy is (1) short-term covariance matrix>(2) finequantization channel matrix>(3) long-term covariance matrix>(4) PMI>(5)rough quantization channel matrix in descending order of accuracy.Accordingly, in the case of determining content of channel stateinformation so that accuracy of feedback to a serving cell is higherthan accuracy of feedback to a coordinated cell, combinations of contentof CSI to feed back to a serving cell and content of CSI to feed back toa coordinated cell are as shown in Table 3 (circles show availablecombinations.)

When there is a plurality of options in the combination of content ofCSI to feed back to a serving cell and content of CSI to feed back to acoordinated cell, it is preferable to determine the CSI content by amethod (first method) of selecting a combination that maximizes feedbackaccuracy, another method (second method) of determining the content ofCSI to feed back to a coordinated cell based on reception qualitynotified from a user terminal or the like.

The first method will be described with reference to FIG. 3. Forexample, when the content of CSI to feed back to a serving cell is (1)the short-term covariance matrix, the option of the content of CSI tofeed back to a coordinated cell is (2) the fine quantization channelmatrix, (3) long-term covariance matrix, (4) PMI or (5) roughquantization channel matrix. In this case, in the first method, thecontent of CSI to feed back to a coordinated cell is set for (2) thefine quantization channel matrix with the highest accuracy among theoptions. Further, when the content of CSI to feed back to a serving cellis (2) the fine quantization channel matrix, the content of CSI to feedback to a coordinated cell is (3) the long-term covariance matrix, (4)PMI or (5) rough quantization channel matrix. In this case, in the firstmethod, the content of CSI to feed back to a coordinated cell is set for(3) the long-term covariance matrix with the highest accuracy among theoptions. Furthermore, when the content of CSI to feed back to a servingcell is (3) the long-term covariance matrix, the content of CSI to feedback to a coordinated cell is (4) the PMI or (5) rough quantizationchannel matrix. In this case, in the first method, the content of CSI tofeed back to a coordinated cell is set for (4) the PMI with the highestaccuracy between the options.

In addition, it is possible to determine the content of CSI to feed backto a serving cell as appropriate corresponding to reception qualityinformation of a user terminal (for example, determine the CSI contentby a threshold determination as described above). Further, as the cellquality information, there are RSRP (Reference Signal Received power),RSRQ (Reference Signal Received Quality), SINR (Signal to Interferenceplus Noise Ratio) and the like.

In the second method, the content of CSI to feed back to a coordinatedcell is associated with reception quality in a user terminal, and isdetermined corresponding to the reception quality. For example, whenthere is a plurality of options in the combination of content of CSI fora serving cell and content of CSI for a coordinated cell, a plurality ofthresholds is beforehand set in association with reception quality, andthe content of CSI for a coordinated cell is determined corresponding toreception quality of the coordinated cell. For example, when the contentof CSI to feed back to a serving cell is (1) the short-term covariancematrix, the option of the content of CSI to feed back to a coordinatedcell is (2) the fine quantization channel matrix, (3) long-termcovariance matrix, (4) PMI or (5) rough quantization channel matrix. Inthis case, three different thresholds are set in association with thereception quality (first threshold, second threshold, and thirdthreshold in descending order of reception quality). When the receptionquality exceeds the first threshold, the content of CSI to feed back toa coordinated cell is set for (2) the fine quantization channel matrix.When the reception quality is the first threshold or less whileexceeding the second threshold, the content of CSI to feed back to acoordinated cell is set for (3) the long-term covariance matrix. Whenthe reception quality is the second threshold or less while exceedingthe third threshold, the content of CSI to feed back to a coordinatedcell is set for (4) the PMI. When the reception quality is the thirdthreshold or less, the content of CSI to feed back to a coordinated cellis set for (5) the rough quantization channel matrix. Similarly, whenthe content of CSI to feed back to a serving cell is (2) the finequantization channel matrix, the option of the content of CSI to feedback to a coordinated cell is (3) the long-term covariance matrix, (4)PMI or (5) rough quantization channel matrix. When the reception qualityis the first threshold or less while exceeding the second threshold, thecontent of CSI to feed back to a coordinated cell is set for (3) thelong-term covariance matrix. When the reception quality is the secondthreshold or less while exceeding the third threshold, the content ofCSI to feed back to a coordinated cell is set for (4) the PMI. When thereception quality is the third threshold or less, the content of CSI tofeed back to a coordinated cell is set for (5) the rough quantizationchannel matrix. Similarly, when the content of CSI to feed back to aserving cell is (3) the long-term covariance matrix, the option of thecontent of CSI to feed back to a coordinated cell is (4) the PMI or (5)rough quantization channel matrix. When the reception quality is thesecond threshold or less while exceeding the third threshold, thecontent of CSI to feed back to a coordinated cell is set for (4) thePMI. When the reception quality is the third threshold or less, thecontent of CSI to feed back to a coordinated cell is set for (5) therough quantization channel matrix. Thus, in the second method, thecontent of CSI is determined so that feedback accuracy is lower as thereception quality is lower.

In addition, it is possible to determine the content of CSI to feed backto a serving cell as appropriate corresponding to reception qualityinformation of a user terminal (for example, determine the CSI contentby the threshold determination as described above). Further, as the cellquality information, there are RSRP, RSRQ, SINR and the like.

Herein, a procedure of control according to the above-mentioned secondmethod will be described with reference to FIG. 4. FIG. 4 is a sequencediagram illustrating the procedure between a radio base stationapparatus and a user terminal.

First, a user terminal measures quality information (for example, RSRP)for each cell to perform Coordinated Multi-Point (ST11). The userterminal notifies a radio base station apparatus of the qualityinformation by higher layer signaling, for example.

Next, the radio base station apparatus makes a threshold determinationas described above on the reception quality (ST12). Next, the radio basestation apparatus determines the content of CSI of the serving cell andcoordinated cell as described above based on a result of the thresholddetermination (ST13). Subsequently, the radio base station apparatusnotifies the user terminal of the CSI content information, for example,by higher layer signaling (RRC signaling, broadcast information).

Next, the user terminal generates CSI with the CSI content informationnotified from the radio base station apparatus (ST14). Then, the userterminal transmits thus generated CSI to radio base station apparatusesof cells that perform Coordinated Multi-Point transmission for each cellas feedback (ST15). According to such control according to theinvention, it is possible to achieve a maximum 30 percentage reductionin overhead of feedback information in a state in which throughput inapplying CoMP is maintained.

A radio communication system according to the Embodiment of theinvention will specifically be described below. FIG. 5 is an explanatorydiagram of a system configuration of the radio communication systemaccording to this Embodiment. In addition, the radio communicationsystem as shown in FIG. 5 is a system including the LTE system or SUPER3G, for example. In the radio communication system, used is carrieraggregation for integrating a plurality of base frequency blocks with asystem band of the LTE system as a unit. Further, the radiocommunication system may be called IMT-Advanced or may be called 4G.

As shown in FIG. 5, the radio communication system 1 includes radio basestation apparatuses 20A and 20B, and a plurality of first and seconduser terminals 10A and 10B that communicate with the radio base stationapparatuses 20A and 20B, and is comprised thereof. The radio basestation apparatuses 20A and 20B are connected to an upper stationapparatus 30, and the upper station apparatus 30 is connected to a corenetwork 40. Further, the radio base station apparatuses 20A and 20B aremutually connected by wired connection or wireless connection. The firstand second user terminals 10A and 10B are capable of communicating withthe radio base station apparatuses 20A and 20B in cells C1 and C2. Inaddition, for example, the upper station apparatus 30 includes an accessgateway apparatus, radio network controller (RNC), mobility managemententity (MME), etc., but is not limited thereto. In addition, amongcells, when necessary, a plurality of base stations performs control ofCoMP transmission.

The first and second user terminals 10A and 10B include LTE terminalsand LTE-A terminals, and are described as first and second userterminals unless otherwise specified in the following description.Further, for convenience in description, the description is given whileassuming that equipments that perform radio communications with theradio base station apparatuses 20A and 20B are the first and second userterminals 10A and 10B, and more generally, the equipments may be userequipments (UEs) including user terminals and fixed terminals.

In the radio communication system 1, as a radio access scheme, OFDMA(Orthogonal Frequency Division Multiple Access) is applied in downlink,while SC-FDMA (Single Carrier-Frequency Division Multiple Access) isapplied in uplink, and the uplink radio access scheme is not limitedthereto. OFDMA is a multicarrier transmission scheme for dividing afrequency band into a plurality of narrow frequency bands (subcarriers),and mapping data to each subcarrier to perform communications. SC-FDMAis a single-carrier transmission scheme for dividing the system bandinto bands comprised of a single or consecutive resource blocks for eachterminal so that a plurality of terminals uses mutually different bands,and thereby reducing interference among the terminals.

The downlink communication channels have the PDSCH (Physical DownlinkShared Channel) as a downlink data channel shared among the first andsecond user terminals 10A and 10B, and downlink L1/L2 control channels(PDCCH, PCFICH, PHICH). Transmission data and higher control informationis transmitted on the PDSCH. Scheduling information of the PDSCH andPUSCH and the like is transmitted on the PDCCH (Physical DownlinkControl Channel). The number of OFDM symbols used in the PDCCH istransmitted on the PCFICH (Physical Control Format Indicator Channel).ACK/NACK of HARQ to the PUSCH is transmitted on the PHICH (PhysicalHybrid-ARQ Indicator Channel).

The uplink communication channels have the PUSCH (Physical Uplink SharedChannel) as an uplink data channel shared among the user terminals, andthe PUCCH (Physical Uplink Control Channel) that is a control channel inuplink. Transmission data and higher control information is transmittedon the PUSCH. Further, on the PUCCH is transmitted downlink receptionquality information (CQI), ACK/NACK and the like.

Referring to FIG. 6, described is the entire configuration of the radiobase station apparatus according to this Embodiment. In addition, theradio base station apparatuses 20A and 20B have the same configuration,and therefore, are described as the radio base station apparatus 20.Further, the first and second user terminals 10A and 10B described lateralso have the same configuration, and therefore, are described as theuser terminal 10. The radio base station apparatus 20 is provided withtransmission/reception antennas 201, amplifying sections 202,transmission/reception sections (notification section) 203, basebandsignal processing section 204, call processing section 205 andtransmission path interface 206. The transmission data to transmit fromthe radio base station apparatus 20 to the user terminal in downlink isinput to the baseband signal processing section 204 via the transmissionpath interface 206 from the upper station apparatus 30.

The baseband signal processing section 204 performs, on the downlinkdata channel signal, PDCP layer processing, segmentation andconcatenation of the transmission data, RLC (Radio Link Control) layertransmission processing such as transmission processing of RLCretransmission control, MAC (Medium Access Control) retransmissioncontrol e.g. HARQ transmission processing, scheduling, transmissionformat selection, channel coding, Inverse Fast Fourier Transform (IFFT)processing and precoding processing. Further, on a signal of thePhysical Downlink Control Channel that is a downlink control channel,the section 204 also performs transmission processing of channel coding,Inverse Fast Fourier Transform and the like.

Further, the baseband signal processing section 204 notifies userterminals 10 connected to the same cell of control information for eachuser terminal 10 to perform radio communications with the radio basestation apparatus 20 on the broadcast channel. For example, theinformation for communications in the cell includes the system bandwidthin uplink or downlink, identification information (Root Sequence Index)of a root sequence to generate a signal of a random access preamble onthe PRACH (Physical Random Access Channel), etc.

The transmission/reception section 203 converts the frequency of thebaseband signal output from the baseband signal processing section 204into a radio frequency band. The amplifying section 202 amplifies aradio frequency signal subjected to frequency conversion to output tothe transmission/reception antenna 201. In addition, thetransmission/reception section 203 constitutes reception means forreceiving an uplink signal including information of a phase differenceamong a plurality of cells and PMI, and transmission means forperforming Coordinated Multi-Point transmission on a transmissionsignal.

Meanwhile, with respect to signals transmitted from the user terminal 10to the radio base station apparatus 20 in uplink, a radio frequencysignal received in the transmission/reception antenna 201 is amplifiedin the amplifying section 202, subjected to frequency conversion in thetransmission/reception section 203, thereby converted into a basebandsignal, and is input to the baseband signal processing section 204.

The baseband signal processing section 204 performs FFT processing, IDFTprocessing, error correcting decoding, reception processing of MACretransmission control, and reception processing of RLC layer and PDCPlayer on transmission data included in the baseband signal received inuplink. The decoded signal is transferred to the upper station apparatus30 via the transmission path interface 206.

The call processing section 205 performs call processing such as settingand release of the communication channel, status management of the radiobase station apparatus 20, and management of radio resources.

FIG. 7 is a block diagram illustrating a configuration of the basebandsignal processing section in the radio base station apparatus as shownin FIG. 6. The baseband signal processing section 204 is mainlycomprised of a layer 1 processing section 2041, MAC processing section2042, RLC processing section 2043, and CSI content determining section2044.

The layer 1 processing section 2041 mainly performs processingconcerning the physical layer. For example, on a signal received inuplink, the layer 1 processing section 2041 performs processing such aschannel decoding, Discrete Fourier Transform (DFT), frequency demapping,Inverse Fast Fourier Transform (IFFT), and data demodulation. Further,on a signal to transmit in downlink, the layer 1 processing section 2041performs processing such as channel coding, data modulation, frequencymapping and Inverse Fast Fourier Transform (IFFT).

The MAC processing section 2042 performs processing such asretransmission control in the MAC layer on the signal received inuplink, scheduling in uplink/downlink, selection of a transmissionformat of PUSCH/PDSCH, and selection of a resource block of PUSCH/PDSCH.

On packets received in uplink and packets to transmit in downlink, theRLC processing section 2043 performs segmentation of packets,concatenation of packets, retransmission control in the RLC layer andthe like.

The CSI content determining section 2044 determines content of CSI tofeed back to the serving cell and content of CSI to feed back to thecoordinated cell. The CSI content determining section 2044 determinesthe content of channel state information so that the content of CSI tofeed back to the serving cell is different from the content of CSI tofeed back to the coordinated cell, more specifically, accuracy offeedback to the serving cell is higher than accuracy of feedback to thecoordinated cell. By this means, it is possible to reduce overhead ofCSI feedback without decreasing throughput. Further, when there is aplurality of options in the combination of content of CSI to feed backto the serving cell and content of CSI to feed back to the coordinatedcell, the CSI content determining section 2044 determines the CSIcontent by the method (first method) of selecting a combination thatmaximizes feedback accuracy, or the method (second method) ofdetermining the content of CSI to feed back to a coordinated cell basedon reception quality notified from a user terminal. For example, the CSIcontent information is notified to the user terminal by higher layersignaling.

The entire configuration of the user terminal according to thisEmbodiment will be described next with reference to FIG. 8. The LTEterminal and the LTE-A terminal have the same configuration of principalpart of hardware, and are not distinguished to describe. The userterminal 10 is provided with transmission/reception antennas 101,amplifying sections 102, transmission/reception sections (receptionsection) 103, baseband signal processing section 104 and applicationsection 105.

With respect to data in downlink, a radio frequency signal received inthe transmission/reception antenna 101 is amplified in the amplifyingsection 102, subjected to frequency conversion in thetransmission/reception section 103, and is converted into a basebandsignal. The baseband signal is subjected to FFT processing, errorcorrecting decoding, reception processing of retransmission control,etc. in the baseband signal processing section 104. Among the data indownlink, the transmission data in downlink is transferred to theapplication section 105. The application section 105 performs processingconcerning layers higher than the physical layer and MAC layer and thelike. Further, among the data in downlink, the broadcast information isalso transferred to the application section 105.

Meanwhile, with respect to transmission data in uplink, the applicationsection 105 inputs the data to the baseband signal processing section104. The baseband signal processing section 104 performs mappingprocessing, transmission processing of retransmission control (HARQ),channel coding, DFT processing and IFFT processing. Thetransmission/reception section 103 converts the frequency of thebaseband signal output from the baseband signal processing section 104into a radio frequency band. Then, the amplifying section 102 amplifiesthe radio frequency signal subjected to frequency conversion to transmitfrom the transmission/reception antenna 101. In addition, thetransmission/reception section 103 constitutes transmission means fortransmitting information of a phase difference, information of aconnected cell, selected PMI and the like to radio base stationapparatuses eNBs of a plurality of cells and reception means forreceiving a downlink signal.

FIG. 9 is a block diagram illustrating a configuration of the basebandsignal processing section in the user terminal as shown in FIG. 8. Thebaseband signal processing section 104 is mainly comprised of a layer 1processing section 1041, MAC processing section 1042, RLC processingsection 1043, feedback information generating section 1044, and qualitymeasuring section 1045.

The layer 1 processing section 1041 mainly performs processingconcerning the physical layer. For example, on a signal received indownlink, the layer 1 processing section 1041 performs processing suchas channel decoding, Discrete Fourier Transform (DFT), frequencydemapping, Inverse Fast Fourier Transform (IFFT), and data demodulation.Further, on a signal to transmit in uplink, the layer 1 processingsection 2041 performs processing such as channel coding, datamodulation, frequency mapping and Inverse Fast Fourier Transform (IFFT).

The MAC processing section 1042 performs retransmission control (HARQ)in the MAC layer on the signal received in downlink, analysis ofdownlink scheduling information (identification of a transmission formatof the PDSCH, identification of a resource block of the PDSCH) and thelike. Further, the MAC processing section 1042 performs processing suchas MAC retransmission control on a signal to transmit in uplink, andanalysis of uplink scheduling information (identification of atransmission format of the PUSCH, identification of a resource block ofthe PUSCH).

On packets received in downlink and packets to transmit in uplink, theRLC processing section 1043 performs segmentation of packets,concatenation of packets, retransmission control in the RLC layer andthe like.

The feedback information generating section 1044 generates CSI (feedbackinformation). As the CSI, there are CSI (PMI, CDI, CQI) for each cell,inter-cell CSI (phase difference information, amplitude differenceinformation), RI (Rank Indicator) and the like. When the CSI(particularly, CDI) is (1) the short-term covariance matrix, the optionof content of CSI to feed back to the coordinated cell is defined by (2)the fine quantization channel matrix, (3) long-term covariance matrix,(4) PMI or (5) rough quantization channel matrix. The feedbackinformation generating section 1044 generates the CSI according to theCSI content information notified from the radio base station apparatus.In other words, the feedback information generating section 1044generates the CSI (CDI) defined so that accuracy of feedback to theserving cell is higher than accuracy of feedback to the coordinatedcell. These pieces of CSI are transmitted to the radio base stationapparatus as feedback on the PUCCH and PUSCH.

The quality measuring section 1045 measures quality information for eachcell to perform CoMP transmission. As the quality information, there areRSRP, RSRQ, SINR and the like. The quality information is notified tothe radio base station apparatus by higher layer signaling.

In the radio communication system having the above-mentionedconfiguration, first, the quality measuring section 1045 in the userterminal measures quality information (for example, RSRP) for each cellto perform CoMP transmission. For example, the user terminal notifiesthe radio base station apparatus of the quality information by higherlayer signaling.

Next, the CSI content determining section 2044 in the radio base stationapparatus determines CSI content based on reception quality notifiedfrom the user terminal. At this point, the CSI content determiningsection 2044 determines the content of channel state information so thatthe content of CSI to feed back to the serving cell is different fromthe content of CSI to feed back to the coordinated cell, morespecifically, accuracy of feedback to the serving cell is higher thanaccuracy of feedback to the coordinated cell. Subsequently, the radiobase station apparatus notifies the user terminal of the CSI contentinformation, for example, by higher layer signaling.

In addition, when there is a plurality of options in the combination ofcontent of CSI to feed back to the serving cell and content of CSI tofeed back to the coordinated cell, the CSI content determining section2044 selects a combination that maximizes feedback accuracy, ordetermines the content of CSI to feed back to the coordinated cell basedon the reception quality notified from the user terminal.

Next, the feedback information generating section 1044 in the userterminal generates the CSI (CDI) with the CSI content notified from theradio base station apparatus. Then, thus generated CSI (CDI) istransmitted to radio base station apparatuses of cells to perform CoMPtransmission as feedback for each cell together with the other CSI. Bythis means, it is possible to achieve reductions in overhead of feedbackinformation in a state in which throughput in applying CoMP transmissionis maintained.

In the above-mentioned descriptions, the present invention isspecifically described using the above-mentioned Embodiment, but it isobvious to a person skilled in the art that the invention is not limitedto the Embodiment described in the Description. The invention is capableof being carried into practice as modified and changed aspects withoutdeparting from the subject matter and scope of the invention defined bythe descriptions of the scope of the claims. Accordingly, thedescriptions of the Description are intended for illustrativeexplanation, and do not have any restrictive meaning to the invention.

The present application is based on Japanese Patent Application No.2011-243023 filed on Nov. 7, 2011, entire content of which is expresslyincorporated by reference herein.

The invention claimed is:
 1. A radio communication system comprising: aplurality of radio base station apparatuses; and a user terminalconfigured to be able to perform Coordinated Multi-Point (CoMP)transmission/reception with the plurality of radio base stationapparatuses, wherein the radio base station apparatus includes adetermination section that determines content of channel stateinformation to transmit to a serving cell to perform CoMP transmissionas feedback and content of channel state information to transmit to acoordinated cell as feedback such that accuracy of feedback to theserving cell is higher than accuracy of feedback to the coordinatedcell, and a notification section that notifies the user terminal of thecontent of channel state information, and the user terminal includes ageneration section that generates feedback information based on contentof channel state information notified from the radio base stationapparatus, and a transmission section that transmits the feedbackinformation to radio base station apparatuses of cells that perform CoMPtransmission, wherein, when there is a plurality of options on thecontent of channel state information to feed back to the coordinatedcell, the determination section selects an option of the content ofchannel state information that maximizes the accuracy of feedback to thecoordinated cell.
 2. The radio communication system according to claim1, wherein for the coordinated cell, the determination sectiondetermines the content of channel state information such that feedbackaccuracy is lower as reception quality is lower.
 3. The radiocommunication system according to claim 1, wherein the plurality ofoptions includes a short-term covariance matrix, a fine quantizationchannel matrix, a long-term covariance matrix, a precoding matrixindicator and a rough quantization channel matrix.
 4. The radiocommunication system according to claim 1, wherein the CoMP transmissionis Joint transmission type CoMP transmission.
 5. A radio base stationapparatus in a radio communication system provided with a plurality ofradio base station apparatuses, and a user terminal configured to beable to perform Coordinated Multi-Point (CoMP) transmission/receptionwith the plurality of radio base station apparatuses, the radio basestation comprising: a determination section that determines content ofchannel state information to transmit to a serving cell to perform CoMPtransmission as feedback and content of channel state information totransmit to a coordinated cell as feedback such that accuracy offeedback to the serving cell is higher than accuracy of feedback to thecoordinated cell; and a notification section that notifies the userterminal of the content of channel state information, wherein, whenthere is a plurality of options on the content of channel stateinformation to feed back to the coordinated cell, the determinationsection selects an option of the content of channel state informationthat maximizes the accuracy of feedback to the coordinated cell.
 6. Auser terminal in a radio communication system provided with a pluralityof radio base station apparatuses, and the user terminal configured tobe able to perform Coordinated Multi-Point (CoMP) transmission/receptionwith the plurality of radio base station apparatuses, the user terminalcomprising: a generation section that generates feedback informationbased on content of channel state information notified from a radio basestation apparatus; and a transmission section that transmits thefeedback information to radio base station apparatuses of cells thatperform CoMP transmission, wherein the content of channel stateinformation is determined such that accuracy of feedback to a servingcell is higher than accuracy of feedback to a coordinated cell, and whenthere is a plurality of options on the content of channel stateinformation to feed back to the coordinated cell, an option of thecontent of channel state information is selected that maximizes theaccuracy of feedback to the coordinated cell.
 7. A radio communicationmethod in a radio communication system provided with a plurality ofradio base station apparatuses, and a user terminal configured to beable to perform Coordinated Multi-Point (CoMP) transmission/receptionwith the plurality of radio base station apparatuses, the radiocommunication method comprising: in a radio base station apparatus,determining content of channel state information to transmit to aserving cell to perform CoMP transmission as feedback and content ofchannel state information to transmit to a coordinated cell as feedbacksuch that accuracy of feedback to the serving cell is higher thanaccuracy of feedback to the coordinated cell; in the radio base stationapparatus, notifying the user terminal of the content of channel stateinformation; in the user terminal, generating feedback information basedon the content of channel state information notified from the radio basestation apparatus; and in the user terminal, transmitting the feedbackinformation to radio base station apparatuses of cells that perform CoMPtransmission, wherein in the determining step, when there is a pluralityof options on the content of channel state information to feed back tothe coordinated cell, the radio base station apparatus selects an optionof the content of channel state information that maximizes the accuracyof feedback to the coordinated cell.